locatedb(5) File Formats Manual locatedb(5)
NAME
locatedb - front-compressed file name database
DESCRIPTION
This manual page documents the format of file name databases for the GNU
version of locate. The file name databases contain lists of files that
were in particular directory trees when the databases were last updated.
There can be multiple databases. Users can select which databases locate
searches using an environment variable or command line option; see
locate(1). The system administrator can choose the file name of the
default database, the frequency with which the databases are updated, and
the directories for which they contain entries. Normally, file name
databases are updated by running the updatedb program periodically,
typically nightly; see updatedb(1).
GNU LOCATE02 database format
This is the default format of databases produced by updatedb. The
updatedb program runs frcode to compress the list of file names using
front-compression, which reduces the database size by a factor of 4 to 5.
Front-compression (also known as incremental encoding) works as follows.
The database entries are a sorted list (case-insensitively, for users'
convenience). Since the list is sorted, each entry is likely to share a
prefix (initial string) with the previous entry. Each database entry
begins with an signed offset-differential count byte, which is the
additional number of characters of prefix of the preceding entry to use
beyond the number that the preceding entry is using of its predecessor.
(The counts can be negative.) Following the count is a null-terminated
ASCII remainder — the part of the name that follows the shared prefix.
If the offset-differential count is larger than can be stored in a signed
byte (±127), the byte has the value 0x80 (binary 10000000) and the actual
count follows in a 2-byte word, with the high byte first (network byte
order). This count can also be negative (the sign bit being in the first
of the two bytes).
Every database begins with a dummy entry for a file called `LOCATE02',
which locate checks for to ensure that the database file has the correct
format; it ignores the entry in doing the search.
Databases cannot be concatenated together, even if the first (dummy)
entry is trimmed from all but the first database. This is because the
offset-differential count in the first entry of the second and following
databases will be wrong.
In the future, the data within the locate database may not be sorted in
any particular order. To obtain sorted results, pipe the output of
locate through sort -f.
slocate database format
The slocate program uses a database format similar to, but not quite the
same as, GNU locate. The first byte of the database specifies its
security level. If the security level is 0, slocate will read, match and
print filenames on the basis of the information in the database only.
However, if the security level byte is 1, slocate omits entries from its
output if the invoking user is unable to access them. The second byte of
the database is zero. The second byte is followed by the first database
entry. The first entry in the database is not preceded by any
differential count or dummy entry. Instead the differential count for
the first item is assumed to be zero.
Starting with the second entry (if any) in the database, data is
interpreted as for the GNU LOCATE02 format.
Old Locate Database format
There is also an old database format, used by Unix locate and find
programs and earlier releases of the GNU ones. updatedb runs programs
called bigram and code to produce old-format databases. The old format
differs from the above description in the following ways. Instead of
each entry starting with an offset-differential count byte and ending
with a null, byte values from 0 through 28 indicate offset-differential
counts from -14 through 14. The byte value indicating that a long
offset-differential count follows is 0x1e (30), not 0x80. The long
counts are stored in host byte order, which is not necessarily network
byte order, and host integer word size, which is usually 4 bytes. They
also represent a count 14 less than their value. The database lines have
no termination byte; the start of the next line is indicated by its first
byte having a value ≤ 30.
In addition, instead of starting with a dummy entry, the old database
format starts with a 256 byte table containing the 128 most common
bigrams in the file list. A bigram is a pair of adjacent bytes. Bytes
in the database that have the high bit set are indexes (with the high bit
cleared) into the bigram table. The bigram and offset-differential count
coding makes these databases 20–25% smaller than the new format, but
makes them not 8-bit clean. Any byte in a file name that is in the
ranges used for the special codes is replaced in the database by a
question mark, which not coincidentally is the shell wildcard to match a
single character.
EXAMPLE
Input to frcode:
/usr/src
/usr/src/cmd/aardvark.c
/usr/src/cmd/armadillo.c
/usr/tmp/zoo
Length of the longest prefix of the preceding entry to share:
0 /usr/src
8 /cmd/aardvark.c
14 rmadillo.c
5 tmp/zoo
Output from frcode, with trailing nulls changed to newlines and count
bytes made printable:
0 LOCATE02
0 /usr/src
8 /cmd/aardvark.c
6 rmadillo.c
-9 tmp/zoo
(6 = 14 - 8, and -9 = 5 - 14)
REPORTING BUGS
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help>
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COPYRIGHT
Copyright © 1994-2022 Free Software Foundation, Inc. License GPLv3+: GNU
GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it. There
is NO WARRANTY, to the extent permitted by law.
SEE ALSO
locatedb(5)
Full documentation <https://www.gnu.org/software/findutils/locatedb>
or available locally via: info locatedb
locatedb(5)
findutils 4.9.0 - Generated Fri Aug 12 16:35:56 CDT 2022